1. Field of the Invention
The present invention relates to a measurement apparatus and measurement method which measure the light intensity distribution, and an exposure apparatus.
2. Description of the Related Art
A method of evaluating the performance of an optical system while it is mounted in a semiconductor exposure apparatus has conventionally been proposed. This evaluation is performed by transferring by exposure a mask pattern onto a wafer coated with a resist, developing the resist, and measuring, using an SEM (Scanning Electron Microscope) or the like, a resist image formed by the development.
However, this evaluation method requires resist coating, development, and measurement processes, so it not only takes a lot of time for evaluation but also entails enormous costs. Under the circumstances, there has been proposed an evaluation method of imaging a mask pattern or measurement pattern on a plane in the air on the level flush with the wafer surface without exposure, and directly measuring the light intensity distribution of the formed aerial image (to be referred to as an aerial image measurement method hereinafter).
In order to measure a light intensity having a size that is, for example, smaller than the wavelength of a light source, a slit scan scheme which scans a slit having a width shorter than the wavelength and measures light transmitted through the slit by a light-receiving element is often adopted in the aerial image measurement method (W. N. Partlo, C. H. Fields and W. G. Oldham, “Direct aerial image measurement as a method of testing high numerical aperture microlithographic lenses”, J. Vac. Sci. Technol. B, Vol. 11, pp. 2686-2691).
The conventional slit scan scheme uses, for example, a slit 54 formed in a light-shielding film 51, as shown in FIG. 10. FIG. 11 is a schematic sectional view showing a measurement apparatus of the slit scan scheme taken along a line A0-B0 in FIG. 10. A line-and-space pattern (to be referred to as an L/S pattern hereinafter) is illuminated and imaged, thereby forming an aerial image 40 having a periodic light intensity distribution. As a certain component of the light which forms the aerial image 40 is transmitted through the slit 54, the transmitted light is transmitted through a transparent substrate 52 which supports the light-shielding film 51, and enters a light-receiving unit 53. The light which has entered the light-receiving unit 53 is photo-electrically converted, and the converted light is output as a slit signal SS. A sensor 50 including the light-shielding film 51, transparent substrate 52, and light-receiving unit 53 is scanned in the x direction (a direction perpendicular to the longitudinal direction of the L/S pattern) by a stage, and the slit signal SS is monitored for each scan and step. The aerial image is measured using the signal monitored by scanning the slit (to be referred to as a slit scan signal hereinafter).
Assume that the period of a fluctuation of the light intensity distribution of the aerial image has shortened in the above-described conventional slit scan scheme. In this case, if the slit longitudinal direction is misaligned with respect to the direction in which the stripes of the L/S pattern of the aerial image extend parallelly, the degree of modulation of the slit scan signal decreases. FIG. 12 shows the longitudinal direction of the slit 54, and the direction in which an aerial image of an L/S pattern in the longitudinal direction is formed. The aerial image 40 has an L/S pattern formed to have stripes parallel to the y direction, and has its light intensity distribution modulated in the x direction at a half period. The direction in which the stripes of the L/S pattern of the aerial image 40 extend parallelly, and the longitudinal direction of the slit 54 formed in the light-shielding film 51 are misaligned in the rotation direction on the x-y plane by an angle θ. The angle θ is ideally zero to obtain a slit scan signal having a high degree of modulation. However, in practice, the angle θ never becomes zero due to alignment errors. When slit scan is performed in this state, the degree of modulation of the slit scan signal becomes lower than that of the aerial image 40. Furthermore, if the slit position is misaligned up to an angle that corresponds to one period of the L/S pattern of the aerial image or more, the amount of light which enters the slit does not change even by scanning the slit. This decreases the degree of modulation of the slit scan signal to nearly zero, so the measurement becomes impossible. An angle θc formed between the slit and the L/S pattern at this time is given by:θc=arcsin(2HP/SL)where SL is the length of the slit in its longitudinal direction, and HP (half pitch) is the half period of a fluctuation of the light intensity distribution of the aerial image. If the angle between the slit and the L/S pattern is smaller than the angle θc, the amount of light which enters the slit can be modulated by scanning the slit. FIG. 13 is a graph obtained by plotting the function between the half pitch HP and the slit length SL, and the ordinate indicates the angle θc. As can be understood from FIG. 13, when the slit length SL is about 50 μm, the angle θc is about 14 mrad for HP=200 nm and is about 2 mrad for HP=45 nm; the latter angle is significantly smaller than the former angle. The smaller the half pitch HP, the smaller the angle θc. That is, to obtain a slit scan signal having a high degree of modulation as the tolerance of a misalignment lowers, an alignment accuracy high enough to allow the misalignment angle θ to be smaller than at least the angle θc is required.
Conventionally, a method of scanning the slit while changing the slit angle, and adjusting alignment so that the scan signal has a highest degree of modulation, for example, has been used. However, this method not only prolongs the alignment adjustment time but also decreases the alignment accuracy because a change in the degree of modulation becomes small as the degree of modulation increases to some extent.